Security firms Avast and Sophos have warned users of their anti-virus programs to avoid installing the latest Windows 7 and Windows 8.1 updates from Microsoft, after discovering that they can cause system hangs and failures to boot.

Following the release of a selection of security and bug fixes for Microsoft's Windows operating system this week as part of the company's scheduled Patch Tuesday release cycle which occurs on the second Tuesday of every month, users began reporting serious problems ranging from failure to boot through to complete system hangs while logging in. The commonality, beside having installed Microsoft's latest patches: The users had anti-virus packages from Avast or Sophos installed.

Both companies have now issued statements - Avast here and Sophos here - confirming that the latest security patches are interacting poorly with selected, typically enterprise-focused versions of their respective anti-virus applications on Windows 7, Windows 8.1, and Windows Server 2008 and 2012 - but not, interestingly, Windows 10, suggesting that Microsoft's quality control may be focusing its efforts on the company's latest operating system rather than all supported releases.

As is becoming common for issues discovered post-release, Microsoft has responded by blocking release of the updates if affected anti-virus software is detected on the systems. For those who have already installed the updates, both Avast and Sophos recommend that they are uninstalled - leaving systems vulnerable to attack through flaws they would otherwise have patched - until Microsoft can release updated versions through the Windows Update system. For those whose systems no longer boot or log in, this may require a repair install of the operating system.